The long term objective is to increase our fundamental understanding of possible reaction pathways in enzymes which contain metallosulfur clusters, such as nitrogenase and hydrogenase. The specific aims are to investigate the scope, utility, and mechanistic features of reactions which occur at bridging sulfido ligands in complexes with the structural core [(C5H5)MoS2]2. In particular, the roles of dimeric molybdenum complexes in promoting the reduction of unsaturated molecules by the sequential addition of electrons and protons, or more generally, of nucleophiles and electrophiles, will be studied. Three types of dimeric systems which vary in their electronic properties will be investigated. They include: 1) anionic dimers in which a sulfido ligand undergoes nucleophilic attack on, or electron transfer to, a nitrogenase-related unsaturated molecule; 2) neutral dimers in which sulfido ligands form an adduct with the unsaturated molecule and thereby activate it toward further reduction; and 3) cationic dimers which undergo electrophilic attack on substrates. 4) A final section proposes to explore the synthesis and reactivity of dimeric molybdenum-sulfur systems of other structural types. The studies will extend our understanding of structural and electronic factors which control ligand based reactivity in metal sulfur clusters and will provide a background for assessing the possible reactivity of bridging sulfido ligands in the metallosulfur clusters of enzymes.